Perceptual Attrition of Lexical Tone among L1 Yoruba-speaking Children
in Canada
Saliu Shittu and Anne-Michelle Tessier*
1. Introduction
This paper reports a study of tonal perception and attrition in Yoruba (NigerCongo), among children aged 8-15 and their parents (aged 42-47), all originally
from Nigeria and now living in an English-dominant environment in southern
Alberta, Canada. The study’s goals were to document how these child and adult
speakers perceive tone in their L1 after immersion in an L2 (English) context:
how quickly tonal perception is lost; whether some tones undergo more attrition
than others; and which environmental factors encourage or resist attrition among
child learners. The overall results show a rapid loss of tone perception among
children, and also indicate grammatical asymmetries (e.g. greater accuracy for
High tone) which match previous work on Yoruba tone.
This introductory section includes background in three areas: the facts of
Yoruba tone (1.1), the perceptual acquisition of lexical tone (1.2), and the scant
literature on phonological and tonal attrition (1.3). The paper then introduces the
research questions of our study, the experiments themselves, and their results
and discussion.
1.1 Background on Yoruba tone
Yoruba has three lexical tones, namely (H)igh, (M)id and (L)ow (see Ward,
1952; Bamgbose 1966b; Akinlabi and Lieberman 2000, interalia), which
associate with each and every syllable in a word. In many words a change in the
tone changes the meaning of the word, resulting in minimal pairs or even
quintuples as shown in (1a-e). (For ease of reading, tones are indicated
throughout this paper after the segmental string.)
(1) a. ig͡ba MH
‘calabash’
b. ig͡ba LL
‘time’
c. ig͡ba LH
‘garden egg’
d. ig͡ba MM
‘200’
e. ig͡ba ML
‘climbing rope’
Previous phonological studies of these three basic tones (e.g. Akinlabi 1985;
Pulleyblank 1986; Orie 1997, Akinlabi & Libermann 2000) reveal that these
tones vary in their phonological ‘strength’: H tone is considered the strongest or
most stable, while the M tone is the weakest and most unstable. Bakare
(1995:47) reports that, in addition to the highest fundamental frequency, Yoruba
H tone also has the highest intensity, the highest third formant frequency and the
shortest duration – all salient acoustic characteristics. On the other hand,
*
Department of Linguistics, University of Alberta. [email protected],
[email protected]
evidence for the weakness or instability of Weak tones is provided in Orie
(2006b) and illustrated in (2) below. As seen there, adult English native speakers
at the initial stage of L2 Yoruba tend to use only H and L tones in nouns,
possibly influenced by English stress patterns, at the word- or phrasal levels.
These learners are also observed to identify utterance or word-initial M tone as
H tone (2b,c) while they identify utterance or word-final M tones as L tones
(2d):
(2) a. Yewande
b. Adebayɔ
c. owo
d. sɛgũ




(HHMH)
(MHHL)
(MH)
(HM)
(LHHL)
‘Yoruba name’
(LLHL) or (HLHL)‘Yoruba name’
(HL)
‘money’
(HL)
‘be victorious’
Another Yoruba phenomenon cited as evidence of M tone weakness is observed
when vowel hiatus is resolved by deletion across word boundaries (Pulleyblank
1988; Akinlabi and Lieberman 2000). In such contexts, H and L tones are
usually retained while M tones are deleted:
(3) a. /de ile/ (H MH)
b. /ju iwe/ (M LH)
c. /wa oko/ (H MM)
d. /lu ilu/ (M LL)




[dele]
[juwe]
[woko]
[lulu]
(HH) ‘reach home’
(LH) ‘throw a book’
(HM) ‘come to the farm
(LL) ‘beat the drum’
In derived contexts, some basic Yoruba tones also show ‘allotones’ with a
reduced pitch range, which here we will refer to as tonal contours. A H tone
following an L tone becomes a rising tone as in (4a), while an L tone following
an H tone is realized as a falling tone as in (4b) below.
(4) a. iwe
b. mido
(LH)
(HL)


iwe
mido
(L.LH)1
(H.HL)
With respect to tone patterns and word position, the only relevant Yoruba
tonotactic restriction is that word-initial vowels cannot bear high tone; this was
kept in mind when nonce stimuli were created for the experiments to come.
1.2 Background on tonal acquisition
In early infancy, children are able to discriminate any linguistic tone, whether
they are learning a tonal language or not (see e.g. Mattock and Burnham 2006;
Mattock et al 2008; Liu and Kager 2012). But as with other phonological
language-specific contrasts, early tonal sensitivity changes in the first year of
life. Harrison (2000) found that at 6 months, Yoruba-learning infants perceive
1
This is an informal representation to indicate the rising movement in the second
syllable; in (4a) the shrunk font is meant to indicate a residual L in a transition from the L
of the first syllable to the H of the second..
pitch changes ‘within the minimal domain word [as] do English infants of the
same age’ – meaning that infants discriminate H tone from other tones in a
single nuclear domain, but they fail to discriminate M from L tones. Other
studies of tone language-learning infants demonstrate an ability to discriminate
tonal minimal pairs at around the first year (Tse 1978) and the ability to
discriminate lexical tones better than non-tone language infants (Mattock et al
2008). All of these studies demonstrate that tonal processing, as with other
aspects of phonology, undergoes ‘perceptual reorganization’ (PR): a period
when infants’ sensitivity to native contrasts is maintained or increased while the
sensitivity to non-native contrasts decreases. For lexical tones, this
reorganization occurs between 6 and 9 months (Mattock and Burnham 2006;
Mattock et al 2008) – meaning that by 10 months or later, L1 Yoruba-learning
infants continue to reliably attend to tonal contrasts, while babies exposed to
non-tonal languages like English suppress this perceptual ability and begin to
ignore pitch in their phonological processing of words.
On the production side: while tonal perceptual ability develops early, other
developmental studies (e.g. Ajolore 1974; Orie 2012) find that reliable
production of the Yoruba basic lexical tones occurs starting around 12 months,
with the acquisition of the contour tones – rising (LH) and falling (HL) following later between ages 2;6 – 3;0. This study, however, focuses only on
tonal perception.
1.3 Background on phonological attrition
Contact with a more dominant language, and decreased use of native language,
can lead to linguistic loss (or attrition) of an L1 grammatical system. In studying
language attrition, most studies have discussed a combination of environmental
and grammatical (usually ‘markedness’) factors: see e.g. Dressler (1972);
Andersen (1982); Page and Louden (2003); Campbell and Muntzel (1989);
Seliger and Vago (1991); Bullock and Gerfen (2004).
Very little work has been reported on L1 attrition of lexical tone; still less has
considered the loss of tonal perception among African tonal languages, or
among child attriters. Several studies of Chinese and Taiwanese attrition,
however, have focused specifically on the source and nature of tonal loss (see
Chiung 2003; Luo 2005; Yeh and Lu 2012; Yeh and Tu 2012; Yeh and Lin,
2013.) These studies have reported tonal confusion in their subjects as well as
loss of ability to make tonal contrasts. Yeh and Lu (2012) studied the extent to
which Hakka Low level tone confusion is triggered by attrition, by comparing
three groups of Hakka speakers: young (potential) attriters, young fluent
speakers and older fluent speakers. All the subjects took part in three perception
experiments (AXB discrimination, Identification and Lexical) as well as a
production task. The studies indeed found that fluent speaker produced fewer
tonal errors than attriters in all tasks. In perception, Low level tone was
substituted mostly with the High level tone, presumably due to the similarity in
pitch contours between the two.
2. Research questions of the current study
1. How does perceptual attrition affect Yoruba lexical tone among children?
Which perceptual tasks are most affected?
2. Does the relative ‘strength’ of Yoruba tones predict rates of attrition? Are HL
and LH contours particularly susceptible?
3. Which environmental and external factors best predict attrited children’s
tonal accuracy? Do these factors vary in their effects across tasks?
3. Three perceptual experiments
Three perception experiments were designed for the study using E-prime (v.
2.0): one for AX tonal discrimination using nonce words, one for tonal
identification (again with nonce words), and one involving real lexical items and
picture matching. The tasks were modelled after similar paradigms used by Yeh
and Lu (2012) and Yeh and Lin (2013); see also Liu and Kager (2012).
3.1 Participants
The study recruited a total of 28 child and adult participants, all of them L1
Yoruba users who presently reside in Canada. The child group, with 21
participants ages 8-15, have restricted exposure to their native language. They
rarely speak it at home and never at school, although they still hear it being
spoken, especially at home by their parents. The adult ‘control’ group (7 adults,
mean age of 42 yrs), use their L1 daily, mainly at home and with friends.
To assess the contributions of environmental and input factors to attrition, an
adapted version of the Alberta Language Environment Questionnaire (ALEQ,
Paradis, 2011) was given to parents, written in English, to assess their children’s
language environment The mean and range of the relevant demographic factors
are given in table (5) below, including the questionnaire’s Language Richness
Score. This Score, measured out of 32, takes into account the child’s relative L1
and L2 exposure, including through speaking or listening to parents, other
family members, peers and other sources (e.g. TV, radio, music, etc):
(5)
Mean (range) of results for child participants on adapted ALEQ
years in
age at
age at
language
Canada
test
arrival
richness /32
6.25
12;2
5;11
10.5
(2.6-11.9)
(8;8-15;11) (0-13.1)
(2-18)
3.2 Materials
The nonce word materials (described below) were checked for their conformity
with the language’s syllable system and vowel harmony. In the first two tasks,
nonce words were used to give all participants equal (i.e. zero) experience with
the items, as an attempt to reduce variability caused by these listeners’ very
different language experiences and potentially restricted lexicon. All three sets
of items (nonce and lexical) were recorded by an adult native Yoruba speaker.
They were further verified by another native speaker, naïve to the goals of the
experiments, to ensure that their segments and tones were clear and perceived as
anticipated.
3.3 Procedures
Children and parents were all tested at home. The three tasks were always
administered in the same order of presentation as below; within each task, items
were presented in a fixed but randomized order. Each task was preceded by a
short training session (4-10 trials) in which participants got practice with the
task and received feedback.
3.3.1 Discrimination
In the AX discrimination task, participants listened to a pair of monosyllabic or
disyllabic nonce-word stimuli, one at a time. Then they made a forced choice on
whether the two stimuli had the same Yoruba tones or not:
(6)
Sample AX discrimination nonce word trials
Monosyllabic (N=24)
Disyllabic (N=24)
Tone1 Tone2 Same? Tone 1
Tone 2
lͻ H
dũ H
yes
luwa HM
k͡pͻki HM
wɛ L
ga L
yes
are ML
jeba ML
͡gbo M bɛ L
no
seda LH
kedun ML
ge L
bu H
no
orĩ LL
finju HH
Same?
yes
yes
no
no
3.3.2 Identification
In the identification task, participants listened to a disyllabic or trisyllabic nonce
word, and then identified the last syllable’s tone. Thus, this task had three
choices: H, M or L.
(7)
Sample identification nonce word trials
Disyllabic (n=24)
Trisyllabic (n=24)
Word
Last Tone
Word
Last Tone
͡kpeta LM
M
M
rokalͻ MLM
L
H
wodã HL
bͻlɛmu HLH
H
L
ikͻ LH
ifalɛ LHL
3.3.3 Lexical choice
In the lexical task, participants first heard a real Yoruba word which forms part
of a tonal minimal pair. After hearing the word, they saw two pictures side-by-
side on the screen, which are both named with the segmental content of the
heard word, but differ in their tonal contour. Participants then had to identify
which picture matched the word they had heard. This is all illustrated
schematically in (8); the picture with no tone indicated was the correct answer,
and the other picture is given with its correct tone for the alternate meaning:
(8)
Sample lexical minimal pair trials
Monosyllabic (n=11)
sound
pic1
pic2
[bo]
H
peel
Disyllabic (n=11)
?
sound
1
[odo]
MH
cover
(L)
[ro]
M
pic2
?
2
river
(ML)
2
think
(L)
pic1
mortar
[ak͡pa]
LH
plough
scar
arm
(MH)
For this task, half of the participants saw each picture on the left and the other
half saw it on the right, to ensure that any side bias would not skew results.
4. Results
4.1 Overall accuracy
Table (9) below summarizes child and adult groups’ overall accuracy in the
three experimental tasks, and how many participants per group scored better
than chance on each task. The graph in (10) presents the same group means:
(9)
Task
(# of
trials)
Discrim.
(/48)
Ident.
(/48)
Lexical
(/22)
2
Overall accuracy and no. of better than chance participants
Children (n = 21 or 192)
Adults (n = 7)
Mean
%
Better
Mean
%
Better
correct
correct than
correct
correct than
(SD)
chance? (SD)
chance?
24.5
51%
6/21
35.2
73.5% 6/7
(9.9)
(9.1)
17.6
36.7% 5/19
26
54%
3/7*
(6.4)
(12.9)
11.8
53.8% 5/19
21.4
96%
7/7
(3.7)
(1.9)
Two children dropped out of the study after the first task.
(10) Overall accuracy by group and task
100%
74%
80%
60%
51%
54%
96%
54%
child
37%
40%
adult
20%
0%
discrim
ident
lexical
The clear result is that, despite their frequent home exposure, the school-aged
children in our sample have undergone considerable attrition in their tonal
perception. Only five or six out of the 20-odd children scored above chance
levels on each task (though it was not the same children who succeeded in each
task).
In contrast, the adults who use the language daily (though in a restricted
environment) were more accurate in their tonal perception than the children in
all three tasks, and were fairly skilled at the discrimination and lexical labeling
task. Identification proved the most difficult for both groups; while only 3/7 of
the adults performed better than chance (see * in table 9), it should be noted that
two other adult’s scored one point below the cut-off for better-than-chance
accuracy.
One grammatical factor that proved completely un-informative was word size:
within any of the tasks, items of one, two or three syllables showed near-equal
performance for both groups, so we have collapsed word size in all of our results
unless otherwise indicated. The fact that word size was irrelevant to children’s
performance in particular is striking, as it rules out the possibility that their
attrition is driven by poor short-term tonal memory. In other words, children are
not failing to discriminate and identify tones simply because they cannot
remember them long enough, since longer items do not cause poorer
performance.
4.2 Further results from each task
4.2.1 AX discrimination
In the AX discrimination task, no one tone stood out as better discriminated than
the others. One main, unsurprising result is that pairs of different tones were
more successfully recognized as different than same tones were recognized as
same. The graph in (11) illustrates this advantage for different tone pairs among
both populations (significant for both on two populations on a chi-square test of
the means, p< 0.01)
(11) AX discrimination accuracy by group and answer
100%
80%
60%
79%
66%
56%
45%
child
40%
adult
20%
0%
same tones
diff. tones
4.2.2 Identification
As seen in the overall results, the tonal identification task was the hardest for
both groups, and indeed a couple adults failed to reliably identify tones just as
the children did. This no doubt reflects in part the psychological integrity of
tones as part of the phonological system; labeling the tone of a nonce syllable is
a hard task for speakers, requiring considerable metalinguistic ability.
A notable result is that this task also revealed a High-tone advantage among the
child participants, as seen in (12)’s graph of mean accuracy by tone. For adults,
all three tones were identified at equal, and greater than chance, levels (recall
that chance in this task is 1/3) – but for children, H tones were the only ones
reliably labelled:
(12) Overall identification accuracy by group and tone
100%
80%
60%
52%
45%
40%
54%
33%
57%
33%
Child
Adult
20%
0%
H
M
L
Another child-specific result was that HL and LH sequences did inhibit tonal
recognition, as predicted. The data in (13) compares the correct identification of
H and L tones when in a contour sequence or not: e.g. identification of Low tone
in final [HL] compared to final [ML] and [LL]. Chi-squared tests reveal that L
and H tones both were significantly harder to identify in contours than elsewhere
(p < 0.05.)
(13)
Children’s accuracy in H and L tone discrimination
in a contour
in a non-contour
final L
28/114
25%
71/190
37%
final H
25/76
33%
111/228
49%
4.2.3 Lexical task
The overall result of the lexical labeling task was that adults were roughly at
ceiling, while the majority of children were still at chance. Thus it is clear that
the child group’s lexical knowledge of tone had been much more affected by
their time in the English-speaking context than their parental counterparts.
This result indicates a few things. First, it confirms that the adults in the study
have not simply undergone overall L1 Yoruba attrition; when given real Yoruba
words, the adult perceivers were still (almost) perfect at distinguishing minimal
pairs by tone alone. Second, it suggests that children had lost the ability to
reliable encode tonal contrasts in many lexical items. Looking back at the words
used in (8), these included some trials with one very high frequency word like
‘think’ and another rather less frequent like ‘plough’. Even if we are skeptical
that children living in an urban Canadian environment know the word for
‘plough’: if a child at least knows the word for ‘think’, and if its tonal contour is
part of their underlying representation for ‘think’, they would not be at chance at
rejecting the wrong tonal contour for this word. (Given the phonemic state of
Yoruba tone, this would be akin to a young English learning child who might
not know the word ‘zinc’ but would still not accept it as a possible pronunciation
for the word ‘think’.)
There is again a small hint in this data that H tones are better represented in
children’s lexicons. With respect to monosyllabic words alone: H tone words
were accurately labeled 59% of the time, whereas M tone words (the weakest)
were only accurate 49% of the time (significantly different at p=0.05).
4.3 Correlations between ALEQ and task accuracy
Linear mixed-effect models (using the lmer4 function in R) were used to
examine the contribution of children’s ALEQ participant scores of language
input and richness. Each experiment’s accuracy was modelled separately.
Potential fixed effects included age of test (months), age at arrival (months),
months in Canada, birth country (Nigeria or not), and a language richness score
(scored on the ALEQ out of 32); random slopes were used for participants and
items. Due to collinearity established among predictors, we looked for models
which relied either on age of arrival and months in Canada, or on age at test.
With respect to the nonce word tasks – tonal discrimination and identification –
children’s increased accuracy was predicted by older age of arrival and more
months in Canada, but not age at test. Thus, attrition in this domain is
unsurprisingly connected to longer exposure to English and less majority-
language exposure to Yoruba. In the lexical task, however, the two contributing
predictors of greater accuracy were older age both of arrival and at test. This
latter result suggests that even the attriting learners may be continuing to learn
lexical items over time, or at least were doing so soon after arriving in Canada.
It should be noted that language richness scores as computed via the ALEQ did
not significantly predict accuracy – but this may simply indicate that the range
of scores within our sample was too small, as their average L1 (Yoruba) richness
score was only 10/32 (recall table 5).3
5. Overall Discussion
This study’s three experiments revealed considerable phonological attrition
among L1 child learners of Yoruba now living in an L2 English-dominant
context. While these school-aged children have been exposed to L1 Yoruba
from birth, and hear Yoruba spoken by their native speaker parents on a daily
basis, their tonal discrimination and identification are significantly less accurate
than those of their parents, and they struggle to associate the correct tones with
lexical representations even of highly-frequent words.
Despite their extensive attrition, they do however show asymmetric tonal
abilities which mirror previous findings in L1 and L2 Yoruba studies – namely
that High tones are in some sense privileged, revealed through improved
identification rates, and that HL and LH contours reduce the relative pitch range
and so decrease identification accuracy. The study’s examination of
environmental factors found some predictable results – that the children with
less attrition had spent proportionally more time in the L1 context and less in the
L2 context. It also found an important difference between nonce word and
lexically-dependent tasks, in that age at test was not correlated on its own with
tonal identification and discrimination, but that older children were better at
using tone to identify words.
The extent of these children’s tonal attrition may be rather surprising, but it
should be noted that accuracy on these tasks among monolingual Yoruba
speakers (or among children of any language background) is not yet known.
Thus, our next research project is to collect comparable data from children in
both monolingual Yoruba and English contexts. Only by comparing these three
language backgrounds will we be able to comprehensively assess the relative
influence of tonal attrition, task demands and language-independent tonal
processing for the Yoruba children studied here.
3
In addition, accuracy on the nonce word tasks was correlated with birth outside of Nigeria. This at
first seems completely counter-intuitive – but it may simply be an artifact of a few families who
spent some years in Kuwait before arriving in Canada, within an L1 Yoruba speaking community,
and where great emphasis was placed on children learning to speak in Yoruba.
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